The present invention relates to diamond abrasive compacts and cutters, and in particular to such compacts which have improved fracture resistance.
Polycrystalline diamond compacts, cutters, inserts, and various cutting tools (hereinafter referred to as "compacts") are now in wide use both in oil field drill bits and as machine cutting tools. The compacts are fabricated with a polycrystalline diamond layer bonded to a tungsten carbide substrate by the application of high pressure and temperature in a manner well known to those skilled in the art.
Compacts make highly abrasive-resistant cutters for drilling through rock and cutting through other hard material. The compacts are generally cylindrical with the diameter of the polycrystalline material coextensive with the diameter of the substrate, and the polycrystalline cylinder terminating at right angles with an upper cutting surface. The edge or corner formed by the described right angle is widely used as a cutting edge.
The cutting edge of conventional compacts are formed by a number of grinding or lapping processes. One approach is to use electrical discharge grinding to grind the polycrystalline diamond surface of the compact substantially flat followed by centerless grinding of the peripheral edge of the diamond layer. Another approach is to use free abrasive lapping to prepare the polycrystalline diamond surface, followed by centerless grinding to finish the peripheral edge. These processes invariably leave microfractures or cracks and irregularities that can readily be seen at 20.times. magnification at the cutting edge of the compact.
In cutting rock and hard materials such as those commonly encountered in oil field drilling, the sharp edge of the compact is exposed to intermittent cutting forces of a magnitude related to the compressive strength of the rock formation being drilled. When these cutting forces act on the microfractures and irregularities of the compact cutting edge, cracks develop in the polycrystalline diamond layer leading to chipping and spalling of the cutting edge, accelerated edge wear, and in some cases mechanical failure of the compact. This process can further lead to failure of the drilling or cutting implement due to loss of symmetry or balance, and the increased incidence of random abrasive material within the working environment due to the failure of other components.
One approach to minimizing the effects of these irregularities is to chamfer or bevel the corner at some intermediary angle. This allows the applied forces in rock drilling to be spread over a larger area of the compact, making it less likely that a crack would initiate from the microfractures or irregularities found at the compact cutting edge. However, chamfering the cutting edge corner creates new problems. First it requires a significant increase in the cutting forces required to penetrate the worked material. This is due to the increase in degree of attack of the cutting edge of the polycrystalline layer, in effect, making the edge appear dull to the working surface. For example, an unchamfered edge presents a 90 degree edge and an implement chamfered at 45 degrees presents a 135 degree edge. (See FIG. 2 and FIG. 5). Thus, to maintain the same drilling rate, cutting force pressures must be increased to compensate for the lack of a sharp edge in the chamfered compact. The resulting increase in cutting force causes increased abrasion, and a complementary increase in temperatures, all leading to faster wear and shorter tool life.
The effective rake angle becomes more negative with chamfering (-65 degrees compared to only -20 degrees for an unchamfered piece). This similarly requires an increase cutting force to cause failure in the rock. Wear, temperature and friction problems similar to those described above result, in addition to increased drag on the tool.
Further, chamfering decreases the total mass of polycrystalline material of the compact, and this decreases the effective work life of the tool.
Finally, chamfering also creates two new relatively sharp angles which also contain microcracks and irregularities, leading to rapid failure.